This project concerns patients with chronic intraventricular conduction disease. The proposal is a continuation of work done under the sudden death program (NIH contract 71-2478 and grant HL 18794). The study group consists of patients with intact conduction and bundle branch block (540 patients studied and followed at present). The objective is to delineate life history of chronic conduction disease, with specific aim of delineating high risk groups for development of symptomatic bradyarrhythmia, tachyarrhythmia, and sudden death. If such groups can be delineated, significant decrease in morbidity and mortality might be expected with intervention. It is proposed that initial evaluation will allow delineation of high risk groups (as determine by subsequent follow-up). For example, H-V prolongation might correlate with subsequent development of symptomatic trifascicular block. Patients are detected by screening of inpatient and outpatient electrocardiograms. Patients are then evaluated clinically, electrocardiographically (recording of His potentials, measurement of pacing responses and refractory periods and evaluation of sinus node function) and then followed prospectively in conduction disease clinics, with specific goals of detecting significant bradyarrhythmia and sudden death. Portable tape recorder monitoring is used to increase sensitivity of bradyarrhythmia detection. All initial and follow-up data is computerized, allowing analysis with rapid information retrieval, multivariate analysis, and life table methods. Pathological correlations (serial section of conduction system) are performed in patients dying, Dr. Lev's laboratory. In this new grant request, we are in addition proposing restudy of patients at 3 to 5 years after initial study, in attempt to delineate stability or progression of conduction findings. In addition, extensive diagnostic evaluation of patients with apparent primary conduction system disease is proposed, since life history in this group has not been benign in our experience.